hydrogen notes

hydrogen notes - 1 Hydrogen Shriver, Chapter 9 What should...

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Unformatted text preview: 1 Hydrogen Shriver, Chapter 9 What should everyone know about hydrogen? - the lightest element - the most abundant element in the universe (90% of all atoms) - not the most abundant element on Earth- only 1 ppm H 2 in the atmosphere - forms more compounds than any other element - the only atom for which the Schrodinger equation can be solved exactly - highly flammable; reacts explosively with O 2 (with a spark) - carrier of Brnsted acidity - the best candidate to replace oil as our energy carrier (?!*) - melting point 14 K, boiling point 20 K - discovered 1766 by Henry Cavendish, who liberated H 2 by electrolysis of water (he thought it was phlogiston) - when burned, it produced water hence the name (from the Greek, means water former) What is hydrogen, chemically? - an atom with a single proton and a single electron variations: an atom with 1p, 1e and 0, 1 or 2n 1 1 H " H , protium (H + is proton) 1 2 H " D , deuterium, a stable isotope (D + is deuteron) 1 3 H " T , tritium, an unstable isotope (half-life 12.4 years) What do the additional neutrons change, physically? - the boiling points of D 2 /D 2 O are slightly lower than those of H 2 /H 2 O - interesting application: the global isotope thermometer measures D content in polar ice caps - the colder the global temperature, the more the oceans become enriched in D and the more the rain/snow/ice becomes depleted in D - this is how we know what the average global temperature was over the past 200,000 years (paleoclimatology) - the density of heavy ice being greater than that of ordinary water, it will sink What do the additional neutrons change, chemically? - a plant watered with heavy water (D 2 O) will not grow - it is also toxic to animals, becoming lethal at the level of about 50% replacement of the bodys water content 2 Obviously, the chemical implications of that extra neutron are significant, but why ? Dont chemical reactions involve only the electrons, not the nucleons? The X-D bond is stronger than the X-H bond (for any X). consider the molecule H-H: the -bond is formed by the overlap of two 1s wavefunctions (orbitals) constructive interference of the wavefunctions leads to max. e- density along the internuclear axis plot potential energy E as a function of internuclear distance r E r r e D e the depth of the energy well D e is not equal to the bond dissociation energy of H 2 why? if all motion ceased at 0 K, we would be able to locate precisely the positions of the two hydrogen atoms with respect to each other, at a distance r e this would violate the Heisenberg Uncertainty Principle we can approximate the potential energy as that of a harmonic oscillator E = 1 2 k ( r " r e ) 2 where k is the force constant of the bond, r is the internuclear distance the vibrational frequency (nu), in Hz, is " = 1 2 # k where is the reduced mass: = m 1 m 2 m 1 + m 2 3 in a quantum mechanical harmonic oscillator, the vibrational energy is quantized: E " = h # ( "...
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hydrogen notes - 1 Hydrogen Shriver, Chapter 9 What should...

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